1. Field of the Invention
The present invention generally relates to a system for operating in parallel a plurality of non-break power units (also known as the uninterruptible power equipment or uninterruptible power supply) which are inserted separately between a bypass power source and a plurality of input power sources on one hand and a parallel bus on the other hand. This system will also be referred to as the non-break power unit parallel operating system for the convenience of description. More particularly, the present invention is concerned with the non-break power unit parallel operating system in which the number of inter-unit wiring conductors for interconnecting the individual non-break power units as well as complexity of wiring can significantly be reduced.
2. Description of Related Art
The conventional non-break power unit parallel operating system known heretofore includes a plurality of non-break power units inserted separately or independently from one another between a bypass power source and a plurality of input power sources on one hand and a parallel bus on the other hand, wherein a switchgear (molded case circuit breaker or MCCB for short) is inserted on the output side of each of the non-break power units.
Each of the non-break power units is comprised of a converter for converting an AC power fed from an input power source to a DC power, an inverter (INV) for converting the DC power to an AC power, and an AC switch (alternating current switch) connected to the bypass power source. The non-break power unit has two operation modes, i.e., an inverter or INV power supply mode in which the AC switch is opened for supplying the electric power to the parallel bus from the associated input power source by way of the inverter, and a bypass power supply mode in which the AC switch is closed upon stoppage of the inverter to thereby allow the AC power to be supplied straightforwardly to the parallel bus from the bypass power source by way of the AC switch. (For more particulars; reference may have to be made to Japanese Patent Application Laid-Open Publication No. 10527/2002 (JP-A-H14-10527).)
The AC switch incorporated in the non-break power unit has to be turned on or closed in the single unit operation mode in which the non-break power unit to which the AC switch mentioned above belongs is operating, when the operation of the associated inverter stops (when the inverter power supply to the load from the input power source is interrupted or stopped). However, in the parallel redundant operation mode in which a plurality of non-break power units are operating, the AC switch mentioned above must be maintained in the opened or off state when the inverter power supply is performed by the other non-break power unit(s) regardless of stoppage of the inverter power supply through the concerned non-break power unit to which the above-mentioned AC switch belongs.
Consequently, in the conventional parallel operating system disclosed in the Japanese Patent Application Laid-Open Publication No. 10527/2002 (JP-A-H14-10527), the operation described above is realized by wiring e.g. two non-break power units by a pair of signal lines for mutually transferring the inverter power supply state signals between the two non-break power units.
As is apparent from the above, in the conventional non-break power unit parallel operating system, the inverter power supply state signals are mutually transferred between the non-break power units. Accordingly, when the two non-break power units are operated in parallel, there are required two signal lines between the individual non-break power units. Further, in the case where three non-break power units are operated in parallel, six signal lines are required among the non-break power units. In general, when n non-break power units are operated in parallel, there are demanded n(n−1) signal lines, giving rise to a problem that the inter-unit wiring (i.e., wiring of the signal lines between or among the non-break power units) becomes complicate.